1. Field of the Invention
This invention relates to a method for improving the ground to intensify subterranean loose ground or prevent water permeability in a water-permeable stratum, and also to an apparatus for carrying out the method.
2. Description of the Prior Art
In order to prevent spring water occurrence or water leakage in the ground comprising clayey soil, sandy soil or sands and pebbles or to prevent rupture of soil or intensify soils, it has been so far usually practised to inject a grouting agent in the ground through a hollow injection rod such as a boring rod, a strainer pipe, a double pipe, etc. to be solidified. It is not always satisfactory for stably and uniformly improving the ground depending on the soil stratum conditions such as the intensity, water-permeability, etc. of the ground. In other words, it is not always satisfactory for forming a columnar or spherical coagulation with a uniform diameter, for the following reasons.
When the grouting agent is injected according to the conventional methods, the ground is often ruptured in weak region. Once the ground is ruptured, the grouting agent flows along the ruptured surfaces, and consequently the formed coagulation presents irregular cross-sectional shapes. The rupture of ground has been so far presumed to take place, because the infiltration pressure due to the injection of the grouting agent becomes too high relative to the shearing strength of the concerned region. In other words, according to the conventionally accepted theory, the cause for failure to form a uniform coagulation is an occurrence of a hydraulic fracturing phenomenon in the ground due to that a high infiltration pressure is brought about by the injection, which results in forming shear planes in the ground and occurring infiltration of the grouting agent along these planes, so that the vein-like, irregular coagulation is formed. Thus, it is the conventional expedient to inject the grouting agent under a constant pressure so that the infiltration pressure may not exceed the shearing strength of ground. In order to obtain a thorough infiltration up to the desired region, it takes a long time with an economical dissatisfaction.
As a result of extensive studies, the present inventors have found that the conventionally accepted theory is not correct and the hydraulic fracturing phenomenon of the ground appears to be caused mainly by a tensile stress developed in the ground by the injection of the grouting agent, which is excess of the tensile strength of the ground. When the grouting agent is injected under some pressure through the hollow injection rod, the tensile stress rapidly decreases with increasing distance from the center of the hollow injection rod in the radial direction and rapidly increases towards the center to the contrary. That is, in a diagram having a tensile stress on the ordinate and a distance from the center of the hollow injection rod on the abscissa, the tensile stress can be plotted in a concave curve running from the left upside to the right downside (see FIG. 1). According to this finding, it can be assured that the grouting agent must be injected at an injection rate as low as possible in the initial period of injection, and after the infiltration has been made to the region near the center where the excessive tensile stress is liable to form, the injection rate may be increased continuously (see FIG. 2, curve a) or stepwise (see FIG. 2, curve b) in contrast to said curve of tensile stress. This assumption has been proved to be correct through many tests.
When the grouting agent for stabilizing the ground is injected through the hollow injection rod inserted in the ground according to the well known method, the grouting agent is generally gushed or leaked upwards along the periphery of the injection rod, and consequently it is very difficult to form coagulation in the desired region. That is, prevention of water permeability, or intensification or improvement of the ground cannot be thoroughly attained.
In order to solve the problem that the grouting agent gushes or leaks out, a chemical packer has been proposed, which is a gelation product formed by forcedly injecting a flash setting chemical solution between the periphery of the hollow injection rod and the bore wall. However, the strength of the gelation product is too low to withstand the injection pressure of the grouting agent, and the gelation product is liable to be ruptured and loopholes are liable to develop so that the packer effect will be lost and that the grouting agent will gush or leak out through the loopholes of the packer.
Also proposed are a sleeve injection method comprising steps of providing an outer pipe in a bore hole after a casing boring has been made with a rotary boring machine or a rotary percussion boring machine, filling sealing agent into the clearance between the outer pipe and the bore wall, setting a double packer in the outer pipe at a position corresponding to depths of the ground destined to the injection, then supplying a grouting agent under pressure into a space formed by the packer elements through an inner pipe arranged in the outer pipe, making the grouting agent gushing out the space through small holes formed in the outer pipe, rupturing the sealing agent by the gushing grouting agent and injecting the grouting agent through the resulting cracks, a method utilizing a mechanical packer such as a rubber ring arranged to be pressed on the both sides by a screw means so that a portion thereof is circumferentially protruded beyond the outer surface of the rod or such as an air packer inflatable by compressed air so as to seal the clearance between the periphery of the hollow injection rod and the boring wall. However, these methods need the casing boring in order to set the packer and thus complicate the injection operation, and the maintenance of boring wall is difficult, so that the function of mechanical packer or air packer is deteriorated. That is, no satisfactory packer effect can be obtained.
In a rod injection method using a hollow injection rod, the grouting agent is liable to leak through voids around the hollow injection rod or along the boundary surfaces of coarse grain layers in an unconsolidated ground such as alluvium. In order to prevent grouting agent from leaking out of the injection region, a method for injecting a flash setting grouting agent through a double pipe rod has been proposed, but owing to the short gelation time, the injection of the grouting agent into the ground leads to a vein-like split infiltration, so that the infiltration into the soil grains becomes incomplete.
An injection method using a strainer pipe is not preferable, because the strainer pipe is left in the ground after the injection has been made, and also it is troublesome to insert the strainer pipe in the ground. Also proposed in a composite injection method comprising steps of forming at first the flash setting chemical packer around the double pipe rod above the portion destined for the injection and then infiltrating a long gel-time grouting agent into soil grains. However the strength of the resulting packer is low because of the chemical packer, as described above.